1. Field of the Invention
The present invention relates to a vertical alignment mode liquid crystal display (LCD) device, and more particularly, to a vertical alignment mode LCD device in which an electric field is used to improve the viewing-angle properties.
2. Discussion of the Related Art
Recently with the development of various mobile electronic devices, for example, mobile phones, PDAs, notebook computers, and etc., there has been the increasing demand for flat panel display devices with a thin profile and that are lightweight These flat panel display devices maybe, for example, s a liquid crystal display (LCD), a plasma display panel (PDP), a field emission display (FED), and a vacuum fluorescent display (VFD). The LCD device, in particular, has drawn a great amount of attentions due to the advantageous properties of its mass-production technology, the high picture quality and the good mobility.
The LCD device includes a thin film transistor array substrate; a color filter substrate; and a liquid crystal layer. The thin film transistor array substrate and the color filter substrate are provided opposite to each other, are bonded to each other, and the liquid crystal layer is formed between the two substrates.
The thin film transistor array substrate includes a plurality of pixels arranged in a matrix configuration, wherein each pixel includes a thin film transistor, a pixel electrode and a capacitor. The color filter substrate includes a common electrode; an RGB color filter, which represent various colors; and a black matrix. The common electrode and the pixel electrode apply the electric field to the liquid crystal layer.
An alignment layer having an alignment controlling force is formed on each of facing surfaces of the two substrates, whereby liquid crystal molecules of the liquid crystal layer are aligned in a predetermined direction.
If the electric field is formed between the pixel electrode, formed in each pixel of the thin film transistor array substrate, and the common electrode, formed on an entire surface of the color filter substrate, the aligned liquid crystal molecules are rotated by the dielectric anisotropy. As light is transmitted or blocked by each pixel, letters or images are displayed. This is referred to as the Twisted Nematic (TN) mode, wherein the images are displayed by the operation of the liquid crystal molecules. However, the TN mode LCD device has the disadvantages of having a narrow viewing angle due to the refractive anisotropy of the liquid crystal molecule.
In the TN mode LCD device, the light transmittance is symmetrically distributed in the horizontal direction of the viewing angle. However, the light transmittance is asymmetrically distributed in the vertical direction of the viewing angle. Thus, the inverted image may occur in the vertical direction of the viewing angle due to the asymmetric distribution of the light transmittance in the vertical direction. As a result, the viewing angle becomes narrow.
To overcome this problem of the narrow viewing angle, a multi-domain LCD device has been proposed, for example, a two-domain TN (TDTN) mode or a domain divided TN (DDTN) mode, wherein each pixel is divided into at least two domains, of which main viewing angles are different from each other, to thereby compensate for the viewing angle.
However, for the multi-domain LCD device it is necessary to perform a rubbing process to each domain, so as to obtain the different alignment directions of the liquid crystal molecules in the respective domains. For example, for the two-domain LCD device, the rubbing process is performed to the first domain after blocking the second domain by depositing a photoresist layer on the second domain of the alignment layer, thereby forming the alignment controlling force of the first direction. Then, after removing the photoresist layer from the second domain, another photoresist layer is formed in the first domain, and the second domain is rubbed, to provide the alignment controlling force of the second direction. After rubbing the second domain, the photoresist layer is removed from the first domain by a development process.
For the above-mentioned multi-domain LCD device, one pixel (that is, one substrate) is rubbed by two photolithography processes using the photoresist, thereby increasing the fabrication cost and complicating the process. Also, the development process for the photoresist may have adverse effects on the alignment layer, whereby the picture quality of the LCD device may deteriorate.
To overcome this problem with the multi-domain LCD device, which has a wide-viewing angle structure, a vertical alignment (VA) mode LCD device has been proposed. If a voltage is not applied to the VA mode LCD device, a major axis of liquid crystal molecule is aligned vertically to the alignment layer. If the voltage is applied to the VA mode LCD device, the axis of the liquid crystal molecules is moved to the horizontal direction of the alignment layer since the liquid crystal molecules, which have a negative-type dielectric anisotropy are aligned slantwise by the electric field, so that the light is transmitted through the liquid crystal layer.
In comparison with the TN mode LCD device, the VA mode LCD device has a high contrast ratio and rapid response speed. Furthermore, the VA mode LCD device can realize a wide viewing angle if the alignment direction of the liquid crystal molecules are divided into the various directions and a compensation film is used.
However, it is difficult to align the major axis of liquid crystal molecule perfectly vertical to the alignment layer. Also, the liquid crystal molecule is rotated to one direction. Thus, a color shift may occur according to the viewing angle by the refractive anisotropy of liquid crystal. In order to overcome this problem, a multi-domain VA mode LCD device, which is provided with a pixel including a plurality of domains, has been widely used so that it is possible to compensate for the viewing angle, thereby improving the viewing-angle properties.
FIG. 1A illustrates a related art multi-domain VA mode LCD device. The related art multi-domain VA mode LCD device is provided with a plurality of pixels. However, for convenience of explanation, only one unit pixel will be described as follows.
As shown in FIG. 1A, a pixel is defined by a gate line 1 and a data line 2, wherein the pixel is divided into a plurality of domains. In the plurality of domains, liquid crystal molecules are aligned at the different directions. Thus, the main viewing angle of the domains compensate each other, whereby the viewing-angle properties improve.
Then, a thin film transistor 3 is formed adjacent to a crossing of the gate line 1 and the data line 2, wherein the gate line 1 is supplied with a scanning signal from an external driving circuit, and the data line 2 is supplied with a video signal from the external driving circuit. The thin film transistor 3 is comprised of a gate electrode 4; a semiconductor layer 5 formed on the gate electrode 4; a source electrode 6 and a drain electrode 7. In this state, as the gate electrode 4 is connected with the gate line 1, the scanning signal of the gate line 1 is applied to the gate electrode 4. Also, the semiconductor layer 5 is activated according as the scanning signal is applied to the gate electrode 2. The source and drain electrodes 6 and 7 are formed above the semiconductor layer 5.
Also, the pixel is provided with a pixel electrode 10 connected with the drain electrode 7. That is, as the semiconductor layer 5 becomes activated, the video signal is applied to the pixel electrode 10, whereby the pixel electrode 10 drives liquid crystal (not shown). Also, a common electrode 38 is formed opposite to the pixel electrode 10, thereby forming an electric field therebetween.
The pixel electrode 10 is provided with two of first slits 29, and the common electrode 38 is provided with a second slit 39, wherein the second slit 39 is positioned between the first slits 29 of the pixel electrode 10.
The related art multi-domain VA mode LCD device will be explained in more detail with reference to FIG. 1B.
Referring to FIG. 1B, the related art multi-domain VA mode LCD device includes a first substrate 20, a second substrate 30, and a liquid crystal layer 40 formed between the first and second substrates 20 and 30.
The first substrate 20 includes a gate insulation layer 22; a data line 2 formed on the gate insulation layer 22; a passivation layer 24 formed on the gate insulation layer 22 above the data line 2; and a pixel electrode 10 of a transparent conductive material formed on the passivation layer 24.
The second substrate 30 includes a black matrix 32 which prevents light from being transmitting through a non-display area such as the thin film transistor 3; a color filter layer 34 of red (R), green (G) and blue (B), which represent colors; and a common electrode 38 formed on the color filter layer 34. As the video signal is applied to the multi-domain VA mode LD device, an electric field is formed between the pixel electrode 10 and the common electrode 38.
As shown in FIG. 1B, a second slit 39 is formed in the common electrode 38 of the second substrate 30, whereby the pixel is divided into a plurality of domains. Also, two of first slits 29 are formed in the pixel electrode 10 of the first substrate 20. At this time, the first slits 29 of the pixel electrode 10 are positioned at both sides of the second slit 39 of the common electrode 38, whereby the electric field (E) is distorted between the pixel electrode 10 and the common electrode 38. That is, as show in FIG. 1, the electric field, formed between the pixel electrode 10 and the common electrode 38, is symmetrically formed with respect to the second slit 39 of the common electrode 38 due to the slits 29 and 39. Accordingly, the liquid crystal molecules are symmetrically aligned by the second slit since the liquid crystal molecules are aligned aslant along the electric field. As a result, the liquid crystal molecules are symmetrically aligned in the adjacent domains of the pixel.
To improve the viewing-angle properties in the horizontal direction as well as in the vertical direction, each of the first and second slits 29 and 39 is arranged at an angle of about 45°.
The related art multi-domain VA mode LCD device can improve the viewing-angle properties of a pixel provided with the plurality of domains, but it has the following disadvantages.
As shown in FIG. 2A, the general LCD device is provided with RGB pixels. Recently, a pixel structure including RGBW pixels to improve the luminance and color realization ratio has been proposed, as shown in FIG. 2B.
The LCD device that includes the RGB pixels has a low luminance because most of the light is absorbed in a RGB color filter layer. On the other hand, the light-absorbing ratio can be decreased by changing the pixel structure to the RGBW pixels, whereby the luminance improves. The LCD device using the RGBW pixels includes a pixel having a white (W) color filter layer, which transmits most of light emitted from a backlight unit, whereby the luminance improves.
In the LCD device using the RGB pixel structure, each pixel is formed in shape of a rectangle. In contrast, in case of an LCD device using the RGBW pixel structure, each pixel is formed in shape of a square. For the LCD device using the RGBW pixel structure, each slit provided in the pixel and common electrodes is not formed at an angle of about 45°, so that it is impossible to compensate for the viewing angle in the horizontal and vertical directions, thereby causing the color shift.